DOCS.

108,

109 JULY 1908 79

Please

write

me

what

you

think of

it

because

this is

the

only

encouragement

I have.

What

we are

constructing

first

is

not

that but

this

//////*

///////S/S/S/SSSsss/s

1

approx.

natural size

But the

quantity

of

air called for

is

also

greater

in

a

fast-running pump

with

such

a

tube.

One

always

has

then

a

Hg plug

that

shuts off

a

considerable

amount

of

air.

The rotational

veloc.

is

high

the

cross

section

constant

and

somewhat

larger

only

at

the

air

inlet tube

orifice.

109.

To

August Hagenbach

Bern, 6

July

1908

Highly

esteemed

Professor

Hagenbach:[1]

When

I

made

your

acquaintance

in

Zurich

a

few weeks

ago,[2]

you

told

me

that the

area

of

your

special

interest

is

spectroscopy.[3]

Hence

questions dealing

with

light

interference

surely are

also within

your purview.

I

therefore take the

liberty

of

asking

you a

question

from this

area

with which I have

been

struggling

for

a long

time.-

Suppose a ray

bundle

is decomposed-say

by

partial

reflection

at

a

thin

silver

layer,

into two

completely

coherent

(capable

of

interference)

beams.

Does the

capacity

of

these beams

to

interfere

with

one

another remain

complete

if

each of the

two is

attenuated

to

the

same

extent

by absorption?[4]

According

to

the

customary

theoretical

representation,

which ascribes

the

absorption

to

(friction-)

terms

that

are

proportional

to the

velocity

of the

material carrier of the

motion of

light,

one

would

naturally

have to conclude

that

the

capacity

to

interfere

does

not

diminish. But since

these

terms

can only

be viewed

as a stopgap,

and

since,

in truth,